In many industries, liquid organic compounds are subjected to a filtration or clarification treatment in order to remove any impurity or contaminants therein.
For example, lubricating oil additives such as wear-protecting agents, anti-oxidants, corrosion inhibitors, detergent-dispersants, etc., must be clear and free from any impurities. Many additives comprise metal salts of organic acids, which may contain sulfur and/or phosphorous. Examples of such additives include phenates of alkaline-earth metals, calcium salts of petroleum sulfonic acids commonly referred to as mahogany acids, alkaline-earth metal salts of oil-soluble sulfonic acids, zinc salts of dialkyldithiophosphoric acids, and the like. The process for producing these additives generally comprises reacting the appropriate metal compound, for example zinc oxide, with the organic acid. Additives are often sold as concentrates, including an inert diluent, preferably a, lubricating oil and one or more additives at a concentration higher than that used in the finished lubricating oil composition. These concentrates are easily handled during shipping and storage. They are readily blended with a further amount of lubricating oil to prepare the finished compositions.
The additives and/or the concentrates (hereinafter referred to as additive or oil additive) are subjected to a filtration or clarification treatment. The selection of the filter medium depends on a number of variables but acceptable results are generally achieved by using siliceous material, such as sand, kieselguhr, diatomaceous earth and the like as filter aids. However, the permeability of the filter medium and the filtration rate decrease progressively. Accordingly, the filter medium which has sorbed appreciable amounts of additive has to be discarded and fresh filter medium is employed for further filtration or clarification treatment. The amount of additive sorbed on a spent filter medium may reach 5% or more of the amount of treated additive. In addition to the loss of valuable additive, disposal of additive-laden filter media presents environmental pollution problems that can only be avoided by treatments requiring substantial expense. These factors are detrimental to the economics of the processes for producing lubricating oil additives.
Some attempts have been made in the past to recover the additives sorbed on spent filter medium by using a solvent and separating the solution of additive and solvent from the treated filter medium. The tested solvents were generally light hydrocarbon oils, such as paraffinic hydrocarbons containing from 5 to 8 carbon atoms, or oxygenated compounds, such as alcohols or ethers. However, this technique has several drawbacks. The additive is generally recovered with a poor yield. Moreover, degradation of some additives occurs during the treatment. Consequently, this recovery method has failed to provide an economically acceptable process.
Other processes have also been suggested, but they are specific for one type of additive or the other. For example, oil soluble alkaline earth metal sulfonates may be recovered from spent filter media by treating with an aqueous solution of an alkali metal hydroxide in two steps and at elevated temperature (U.S. Pat. No. 4,501,670).
There is thus a need for an efficient and easily applied technique for recovering an additive sorbed on a filter medium.